This invention relates generally to a method for fabricating high frequency device packages and high frequency packages and in particular to a method for fabricating a high frequency package having coaxial vias and coaxial leads.
As electronic devices and components are operating at higher and higher speeds and increasing frequency ranges, the packaging of the devices becomes an increasing cause of performance degradation. Therefore it is customary to mount semiconductor devices that operate at microwave frequencies directly onto the circuit substrate. This mounting process requires wire bonding or solder-bump mounting. These bonding and mounting techniques have several drawbacks and limitations. For example, the mounting substrate must be rigid for precise mounting and bonding, but such rigid substrates are expensive when large areas are included. In addition, the cost of the microwave device package is further increased due to low wire bond yields, If inverse-die solder bump mounting is used, it may require air cavities trenched onto the substrate to minimize radio frequency (RF) effects on the die circuitry. These trenches, however, are difficult to manufacture. Furthermore, the added inductive reactance of the bond wires requires precise mounting and manual adjustment for repeatable performance. The bond wires also radiate energy causing undesirable energy losses and undesirable cross talk.
There is clearly a need to package such high frequency devices in a surface mount package that can be machine picked and placed onto a printed circuit board. Typically, the interconnections on a printed circuit can maintain controlled impedance and critical connections can be shielded using ground planes and multi layered techniques. It is desirable, however, also to maintain controlled impedance throughout the integrated circuit (IC) package itself. For example, U.S. Pat. No. 5,323,533 to Christian Val (the ""533 Patent) discloses a surface-mounted package whose wire bonds and leads have controlled impedance and a coaxial structure. However, the packaging described in the ""533 Patent still has some limitations in high-frequency applications. In particular, a bond wire connects the electric component to a pad on the package periphery in the ""533 Patent. That pad in turn extends to become the center lead of the package. It is difficult to maintain constant impedance along the bond-wire to pad transition. In addition, it is also difficult to completely shield this connection and it is difficult to maintain the mechanical precision of spacing between those leads, as each lead is extended separately from the main package and those leads tend to bend with mechanical stress of temperature changes.
Thus, it is desirable to provide a high frequency packaging method and high frequency device package and it is to this end that the present invention is directed.
The high frequency device package and packing method in accordance with the invention addresses the above problems. The invention provides a device package with coaxial via connections right through the base of the package. The via structures in the base are precisely fabricated based on photolithography or similar Micro System Technology (MST) techniques well known and currently available in the semiconductor and printed circuit industries. In accordance with the invention, rather than connecting a wire bond to a pad, a wire bond goes directly to the center pin of the coaxial via. The invention includes a method for maintaining the impedance along the entire signal route. An added benefit of this invention is the high density of connections that are feasible by the ability to arrange the coaxial via holes in a two dimensional array, similar to existing ball grid array packages. In accordance with the invention, this package is suitable for high speed digital applications at multi-GHz switching rates.
Thus, in accordance with the invention, an electrical component package is provided that comprises a substantially flat base to accommodate one or more electrical components on its surface and one or more coaxial vias formed in the base for input/output of electrical signals from the electrical component. In accordance with another aspect of the invention, a structure is provided. The structure has an electrical device having at least one conductive pad and a base material to which the electrical device is mounted wherein the base has at least one coaxial via. The structure further comprises a micro-coaxial wire connecting said pad to a conductor in said coaxial via to provide a coaxial signal path from the electrical device to the coaxial via.
In accordance with another aspect of the invention, a method of fabricating coaxial electrical interconnections between a component and a coaxial via is described. In a first step, a wire is placed between said component and a conductive portion of a coaxial via. Next, a conformal coating is applied to the wire and the component with dielectric material. Finally, the dielectric material is conformally coated with a conductive material to form coaxial wires between the component and the coaxial via. In accordance with yet another aspect of the invention, a method of fabricating a base structure with coaxial vias is provided. The method comprises selectively etching an insulating material attached to a flat surface and plating said etched insulating material with a conductive material to form a center conductor for a coaxial via structure.
In accordance with yet another aspect of the invention, a high frequency component package is provided. The package comprises a substantially flat base to accommodate one or more electrical components on its surface and one or more via holes formed in the base to permit signals from the electrical components to pass through the base. Each via hole further comprises a central conductive contact area surrounded by an insulating material ring such that the central conductive contact area, the insulating material ring around the central conductive contact area and the base form a coaxial via connection.
In accordance with another aspect of the invention, a packaged high frequency device is provided. The device comprises a substantially flat base having a high frequency device bonded on its surface wherein the high frequency device has a plurality of bonding pads. The device also has one or more via holes formed in the base to permit signals from the electrical components to pass through the base wherein each via hole further comprises a central conductive contact area surrounded by an insulating material ring such that the central conductive contact area, the insulating material ring around the central conductive contact area and the base form a coaxial via connection. The device also has one or more coaxial bonding wires that connect the bonding pads of the high frequency device to the one or more coaxial via holes in the base wherein each coaxial bonding wire further comprises a conductive bonding wire and a conformal coating of insulating material that surrounds the conductive bonding wire so that the signals are carried by a coaxial structure from the device out through the base.